The present invention generally relates to measuring the thickness of a coating adhered to a substrate and, more particularly, to measuring the thickness of a coating adhered to a substrate by measuring the phase velocity of surface waves propagating through the coating and the substrate and correlating the phase velocity to the thickness of the coating.
In an effort to produce more efficient and economical products, many industries have been producing products by combining various materials in order to gain the benefits of the individual materials. These materials alone typically are not suited for the products, however, when the materials are combined, they offer many benefits over traditional materials. For example, some industries have the need for lighter products that have the integrity of conventional heavier products. Manufacturers produce these lighter products with a substantial proportion of lighter materials and combine the lighter materials with a limited amount of another material that improves the integrity of the lighter material. Thus, the final product is lightweight and has a high degree of integrity. For example, the product may be substantially comprised of a light material, such as aluminum, and may be coated with a limited quantity of a more durable material, such as steel. Accordingly, the final product is lightweight, similar to an aluminum product, and it has the integrity of a product made of steel. In most applications, only a few mils of steel coating is required to substantially improve the integrity of the aluminum product.
One example of the use of coated materials is in the automotive industry. In order to produce more efficient motorized vehicles, some manufacturers are fabricating internal combustion engine blocks from aluminum rather than from more traditional metals such as cast iron. Aluminum is relatively lightweight and aluminum products are relatively easily fabricated, which decreases the weight and manufacturing cost of the engine block and, accordingly, the motorized vehicle. Aluminum, however, is relatively soft and is generally not able to withstand the forces and extreme conditions present in an internal combustion engine. For example, the cylinder walls in the engine block are subject to extreme heat when the fuel is burned and friction as the pistons travel the lengths of the cylinder walls. These conditions on the cylinder walls of an aluminum engine block will cause the engine block to wear and fail after a relatively short period of service.
In order to increase the integrity of the cylinder walls in the aluminum engine block, the cylinder walls are typically coated with a hard material. For example, the cylinder walls may have steel applied to them via a plasma spray. The steel coating increases the durability of the cylinder walls, which in turn, increases the period of service of the aluminum engine block. It has been found that a steel coating of only a few mils will substantially increase the durability of the cylinder walls.
The internal combustion engine, however, will encounter problems if the coating is too thick or too thin. If the coating is too thick, it may impede the movement of the pistons in the cylinders. The impeded movement of the pistons may cause the pistons to score the walls of the cylinder or create excessive heat within the engine, either of which will cause the premature failure of the engine. Furthermore, the application of the plasma coating is expensive and time consuming. Thus, if the coating is thicker than necessary for the operation of the engine, the cost of producing the engine and, thus, the motorized vehicle will increase accordingly.
If, on the other hand, the coating is too thin, friction created by the moving pistons may wear the coating from the cylinder walls and expose the underlying aluminum to the moving pistons. This defeats the purpose of coating the block because the aluminum cylinder walls will directly contact the pistons. As previously set forth, the aluminum alone is soft, thus, the pistons will rapidly wear the aluminum cylinders, which will cause the engine to fail prematurely. In addition, if the coating on one portion of a cylinder wall is very thin, another portion of the cylinder wall may have no coating, which will lead to the rapid failure of the engine.
Further problems will be encountered if the coating is not evenly applied to the cylinder walls. For example, if one side of a cylinder has a thicker coating than the opposite side, the cross-sectional shape of the cylinder will be oblong instead of round. The aforementioned problems will likely be encountered wherein the thickly coated areas of the cylinder walls will impede the movement of the pistons and the thinly coated areas of the cylinder walls will wear prematurely. Either of these problems will cause the premature failure of the engine.
Per the above description, it is crucial that engines are not manufactured from blocks that are improperly coated. Accordingly, the coating thickness and uniformity must be accurately measured so defective blocks are identified and not used in the production of engines. Measuring the coating thickness in a product such as an engine block, however, poses several obstacles, some of which are described below. One example of an obstacle is that the coating thickness must be measured quickly and accurately in a manufacturing environment. If an extended period is required to measure the coating thickness, the price of the engine will increase to reflect the extended measuring period. Another example of an obstacle is that the measuring method must yield results that are easily interpreted by an operator. If the results are difficult to interpret, the operator of the measuring system may indicate that improperly coated blocks have been properly coated, which will result in these defective blocks being used in engines. Yet another example of an obstacle is that the testing must be nondestructive. Destructive testing destroys a portion of the block, which typically increases the cost of the engine and decreases the service period of the engine.
Several methods of measuring the thickness of coatings are presently used in the art; however, they all have drawbacks when they are used to measure a coating adhered to the concave geometry inherent on the surface of a cylinder wall. One method of measuring the thickness of a coating is known in the art as the xe2x80x9cthermal wave method.xe2x80x9d Using this approach, a predetermined portion of the coating is heated. Thermal sensors monitor the heat transmission through the coating and, based on the measured heat transmission, the thickness of the coating may be determined. The equipment required for the thermal wave method is relatively expensive and difficult to use, especially within the confines of a cylinder. Thus, the thermal wave method of measuring the thickness of a coating is not readily applicable for use in the automotive industry.
Another method of measuring the thickness of a coating is achieved by measuring Eddy currents as is known in the art. Using the Eddy current method provides a sensitive measuring technique, however, the Eddy currents may indicate that variations in the coating thickness exist, when in reality, variations in the microstructure of the coating exist. Thus, the Eddy current method may provide confusing information for an operator and, accordingly, is not readily applicable for use on a production line in the automotive industry.
Another method of measuring the thickness of a coating involves using beta particles emitted by radioactive isotopes. This method, however, may pose health risks to the technicians performing the measurements. Accordingly, this method is not preferred for use on a production line.
Therefore, a measuring device and technique are needed that will measure the thickness of a coating adhered to a curved surface, wherein the measuring device and technique are relatively easy to use and are applicable for use in an automotive manufacturing environment.
An apparatus and method for measuring the thickness of a coating adhered to a substrate are disclosed herein. The combination of the coating and the substrate is sometimes referred to herein as the coated material. The apparatus and method are based on principles of surface waves wherein the phase velocity of a surface wave propagating in a coated material is dependent on the thickness of the coating. The apparatus and method are also based on principles of surface waves wherein the coating functions as a bandpass filter for surface waves and wherein a specific frequency of a surface wave undergoes minimal attenuation as it propagates in the coated material. The apparatus and method disclosed herein induce surface waves having fixed and predetermined wavelengths and frequencies into the coated material. Interpolation is used to determine the frequency of surface waves that propagate in the coated material with minimal attenuation. Because the surface waves have a fixed wavelength, the phase velocity of these surface waves is readily determined. This phase velocity is then correlated to a coating thickness.
The measurement apparatus comprises a transmitter, a receiver, and a processor. The transmitter may be of a type known as an xe2x80x9celectromagnetic acoustic transducerxe2x80x9d (EMAT) and serves to induce surface waves into the coated material. The receiver may also be an EMAT that is substantially identical to the transmitter and serves to generate an electric waveform that is representative of the surface waves that propagate in the coated material. The processor is electrically connected to both the transmitter and the receiver. The processor serves to instruct the transmitter as to the frequency of surface waves to induce into the coated material. The processor further serves to analyze the waveform generated by the receiver.
During the measurement process, the processor instructs the transmitter to induce a selected frequency of surface waves into the coated material. The surface waves, accordingly, propagate through the coated material. The receiver, via the EMAT, detects the surface waves after they have propagated through the coated material and generates an electrical representation of the surface waves. The electrical representation of the surface waves is transmitted to the processor for analysis.
The processor analyzes the waveform generated by the receiver to determine the frequency of the surface waves that propagate through the coated material with minimal attenuation. This is accomplished by obtaining the frequency spectrum of the waveform and determining the maximum amplitude of the frequency spectrum and the frequency of the maximum amplitude. The process of inducing surface waves and analyzing the waveform generated by the receiver is repeated for a plurality of frequencies. When a plurality of waveforms generated by the receiver have been analyzed, the processor fits a quadratic equation to the maximum amplitudes based on their corresponding frequencies. The processor then determines the maximum value of the quadratic equation and its corresponding frequency, denoted as fp. The frequency fp is the frequency of surface waves that propagate in the coated material with the least attenuation. Based on the frequency fp, the processor determines the phase velocity of the surface waves having this frequency and correlates this phase velocity to a thickness of the coating.